The requirements made of modern communication standards and of the signal quality of transmitting devices are rising with the growing need for high data rates and increasing mobility. Mobile radio standards that have become customary in the meantime, such as, for example, Universal Mobile Telecommunications System, UMTS, Wideband Code Division Multiple Access, WCDMA, Global System for Mobile Communications, GSM, Enhanced Data rates for GSM Evolution, EDGE, Wireless Local Area Network WLAN or medium-rate Bluetooth, use bandwidth-efficient modulation rates for transmitting high data rates both from a base station to a mobile device and from a mobile device to a base station. Examples of certain modulation rates are Quadrature Phase Shift Keying, QPSK, 8-Phase Shift Keying, 8-PSK or Quadrature Amplitude Modulation, QAM. With these types of modulation, for the purpose of transmitting the data, a so-called carrier signal is modulated both in terms of the phase and in terms of the amplitude.
In this case, a conventional transmitting device comprises a unit for baseband signal processing and a unit for radio frequency signal processing. In this case, the data to be transmitted are preprocessed in the baseband unit in such a way that they can be modulated on to the carrier signal and amplified in the radio frequency unit in order finally to be emitted via an antenna. In this case, in present-day mobile radio standards, the generation of the power required for the transmission signal takes place in the radio frequency range, in particular. Precisely in the case of UMTS it is necessary to provide a plurality of discrete amplification stages which together have to represent a highly linear UMTS amplification path. Said stages are embodied by a plurality of independent setting possibilities in the region of the radio frequency unit. In this case, a control block is fed an analog control signal which is intended to determine the respective amplification stage. In the control block, said control signal is converted into control voltages for the components for amplification and modulation.
FIG. 10 shows one embodiment of a conventional transmitting path, the arrangement comprises a first amplifying device 2 comprising a setting input 20, an I/Q mixer 3a comprising a setting input 30, and a further amplifying device 7 comprising a setting input 70. The arrangement furthermore has an analog control device 5a, comprising setting outputs 52a, 53a and 57a and an analog control input 50a. In this case, the setting inputs of the amplifiers and of the I/Q mixer are connected to the setting outputs of the control device. The setting signals present at the setting outputs are all derived from an analog control signal, which is fed in via the control input 50a, by the analog control device 5a. The influencing of the overall gain curve is achieved by means of diverse programming options in the control device. In this case, it is attempted to simultaneously ensure a minimal current consumption over the control range and also a sufficient linearity.
In the case of linear processing of a signal, the ratio of two values remains constant before and after the processing. In the case of nonlinear processing, by contrast, for example small input values are influenced to a greater extent than large input values, with the result that the value ratio changes. Nonlinear effects occur in amplifiers, for example, if the latter are overdriven, that is to say operated with excessively large signals. In this case, the current consumption arises as well. If amplifiers are underdriven, by contrast, that is to say operated with excessively small signals, their power loss is likewise disproportionately high.
Due to the analog processing of the control signal, the setting signals exhibit a certain dependence with respect to one another. This would not change, moreover, if the control signal were fed to a control device in digital form and converted into an analog control signal by means of a digital-to-analog converter. As a result of the dependence, the arrangement continues to be prohibited from generating arbitrary combinations of setting signals. Therefore, the common influence of the nonlinear characteristic curves of the amplifying elements can be taken into account only inadequately. This holds true particularly with application of a technology based on a Complementary Metal Oxide Semiconductor, CMOS, since the characteristic curves of the transistors used have a highly nonlinear profile. Consequently, the desired aim of a linear overall gain curve with simultaneous power optimal modulation cannot be achieved.